In general, a vehicle braking system is equipped with a brake booster for assisting a brake pedal operation by a driver. To a negative pressure chamber of the brake booster is supplied a negative pressure from a vacuum pump. The brake booster utilizes this negative pressure to boost the power of the brake pedal operation.
In an internal combustion engine, there has been known a blow-by gas recirculation device for recirculating unburnt gas (i.e., blow-by gas) leaking through a gap between a piston and a cylinder during a compression stroke to an intake system of the engine so as to process the unburnt gas. In, for example, a blow-by gas recirculation device disclosed in PTL1, a communication hole is formed so as to allow a crankcase and a chamber inside of a cylinder head cover to communicate with each other; a suction port and an exhaust port of a vacuum pump for supplying a negative pressure to a brake booster are opened to the crankcase and the chamber inside of the cylinder head cover via conduits, respectively; and furthermore, a control valve is disposed on the conduit led to the crankcase in such a manner as to be opened only when a difference in pressure between the crankcase and the chamber inside of the cylinder head cover becomes a predetermined value or higher. In this manner, while in the open state of the control valve which allow recirculating the blow-by gas, by the vacuum pump, the blow-by gas is sucked from the crankcase and is fed to the chamber inside of the cylinder head cover.